Cement paste, mortar and concrete which are obtained by molding cement type admixed and kneaded material are now widely used for various engineering works and for constructing buildings. For constructing such engineering works and buildings various materials such as metal, wood, synthetic resin and glass. Among these materials, molded products of mortar and concrete are the most important, and are used widely as indispensable materials.
Such cement type materials have excellent compression strength and corrosion resistant strength, so that the cement type materials are widely used for constructing a base, an outer wall and a roof of buildings or the like which manifest excellent properties which can not be obtained by using other materials. More particularly, the cement type molded products can be produced at a substantially lower cost than such other materials as metals and synthetic resins. Such low cost can be obtained since the concrete products can readily be obtained by using sand, gravel and water which are natural products.
Although the cement type molded products manufactured by using mortar or concrete can be manufactured at a low cost and have excellent compression strength which can not be obtained when such other materials as metal and synthetic resin are used, the bending strength and pulling strength of the cement type molded products are low. For example, the bending strength of the cement type molded products is low, that is less than 100 kgf/cm.sup.2. For this reason, the field of use of the cement type molded products is limited.
Various investigations have been made for improving the strength of the concrete type molded products. In recent years, a typical method is disclosed in Japanese Patent Publication (JP, B) No. 13,956/1988. According to the method disclosed in this publication, the quantity of a liquid (primary water) used to adhere to a fine aggregate is limited in a narrow range. Then a cement powder is admixed with the wet aggregate so as to cause the cement powder to adhere to the surface of the fine aggregate. Then a quantity of the mixing and kneading water (secondary water) is added for effecting mixing and kneading to obtain a mixture having fluidity and moldability. In this mixture, the cement powder is mixed with the secondary water to form a powder having excellent sliding property, thereby forming a close bonding state which is effective to obtain substantial strength.
However, the strength obtained by this method is the compression strength and its highest value is a most 800 kgf/cm.sup.2 as shown in the Table 8 of the Publication. There is no technical reason for improving the bending strength. The actual bending strength is 70 to 115 kgf/cm.sup.2 as shown in the Table 8, together with the compression strength, such low value of the bending strength does not exceed the prior technical level.
As a method of improving the bending strength of the cement type product, so called resin concrete has been proposed in which a synthetic resin is admixed with the cement type concrete. It has been reported that the bending strength has increased to 1,000 kgf/cm.sup.2 or more. However, according to this method, the reason for increasing the bending strength depends upon the quality and quantity of the incorporated synthetic resin. This reason is fundamentally different than the reason involving the general purpose cement products. Furthermore, the cost of manufacturing the product incorporated with a synthetic resin increases beyond the cost of manufacturing cement products. Moreover, the mixed and kneaded material incorporated with a synthetic resin has a low flowability and moldabity and is difficult to obtain satisfactory products compatible with generally used cement products.
Various types of autoclave curing have been adapted for the cement type products. A typical autoclave curing method utilizes saturated steam having a high temperature exceeding 100.degree. C. and a high pressure. Usually, this autoclave curing utilizes a temperature in a range of 180 to 200.degree. C., and a pressure of 10 to 15 kgf/cm.sup.2. The principal materials utilized in the autoclave products consist of cement, lime and a suitable quantity of a siliceous material. Where a water hardenable mixed and kneaded material is cured in a high pressure auto clave at a temperature of 100.about.200.degree. C., and at a temperature of 10.about.15 kgf/cm.sup.2 such products as fine crystallized gel of CHS(1), tobermorite dicalcium silicate hydrate (C.sub.2 SH), .alpha.C.sub.2 SH, Ca(OH).sub.2 (these products are formed depending upon the mol ratio of lime (CaO) and the siliceous material) and differ from a hydrate obtained by a wet curing under a normal pressure. The above described products obtained by a autoclave curing coexist on independently exist depending upon the mol ratio between lime (CaO) and silica (SiO.sub.2).
The reason of producing various compounds described above is considered as follows.
1 Where the ratio between CaO and SiO.sub.2 becomes 1.about.2 or more, the lime becomes excessive since the quantity of CaO becomes larger than the quantity of SiO.sub.2. As a consequence, the .alpha. C.sub.2 SH coexist with Ca(OH).sub.2 or tobermorite thereby decreasing the strength. PA0 2 When the quantity of CaO becomes lower the 0.7 of the quantity of SiO.sub.2, silica becomes excessive so that stable tobermorite is formed but not yet reacted silica remains as an aggregate. PA0 3 When the ratio between CaO and SiO.sub.2 lies in a range of 0.7 to 1 only the tobermorite is formed. This state is the most advantageous.
However, a case wherein a concrete utilizing normal Portland cement is subjected to an autoclave curing substantially corresponds to the case 1 described above so that contents of .alpha. C.sub.2 SH and Ca(OH).sub.2 become high, while the amount of generated tobermorite becomes small. As a consequence, it is impossible to increase the compression strength and the bending strength. We have confirmed that in a certain case these strengths decrease.
Since such concrete type products have a low bending strength it is necessary to use such reinforcing members as steel frames, iron reinforcing roads, welded metal meshes, wire netting, las netting or the like. Use of many types of such reinforcing members increases the number of the manufacturing steps and the material cost. That greatly decreases the low cost merit of the cement type products. Usually, such reinforcing materials as steel fibers and resin fibers are incorporated into the admixed and kneaded products. Use of such reinforcing materials also increases the manufacturing cost and the member of manufacturing steps generally, since two or more types reinforcing materials described above are used, not only the important feature of low cost decreases, but also moldability and density degrade.
It is well known that the cement type products have a large thickness and weight, an excellent corrosion proof properly and can be manufactured at a low cost, so that the actual field of use of the cement type products is considerably limited despite the advantage of low cost.
Furthermore, since the cross-sectional area necessary to obtain a desired bending strength becomes large with the result that a large quantity of the raw materials must be used, and the weight and shape of the concrete type products become large and complicated. Consequently, it is necessary to incorporate into the concrete type products a substantial amount of iron bars, reinforcing wires, or fibrous members so as to increase the low bending strength. Such incorporation of a substantial amount of the iron bars, reinforcing iron wires or fibrous material increases the manufacturing steps and the cost of the raw materials. Such incorporation of the reinforcing material not only makes it difficult to adjust the mixing and kneading operations, but also impairs the moldability and filling property. Furthermore, the important feature of low cost of the cement type products is greatly decreased.
As is well known that when constructing concrete structures or buildings by using the cement type product it is necessary to use a molding box using plywood plates. This is not advantageous in view of the recent trend of shortage of wood resource, saving of material resource, shortage of the construction time, and saving of manpower. Where a molding box made of mortar or concrete is substituted for a molding box made of laminated plywood, after pouring concrete into the molding box, a desired concrete structure can be obtained so that a molding box removing operation usually made by a skilled workman becomes unnecessary. According to the conventional technique even when a mold box made of mortar is used, the thickness of the molding box becomes large and its weight is also large so that such molding box is not suitable for practical use.
Furthermore, when the cement type products using mortar or concrete is subjected to a high temperature its mechanical strength decreases greatly. Particularly, its bending strength decreases to one half or 1/3 and the compression strength and the modulus of elasticity also decreases to one half. Even though it is known that when a glazing agent is applied to the cement type product, a beautiful product can be obtained, but products that can be sold can not be obtained.